Astronomers offer simple explanation for mysterious X-ray galactic ridge

Press Release From: Northwestern University
Posted: Wednesday, January 9, 2002

EVANSTON, Ill. Astronomers are announcing today that they have imaged in great detail X-ray emission from the mysterious X-ray galactic ridge, in the center of the Milky Way Galaxy. The report is being presented by Farhad Zadeh, professor of physics and astronomy at Northwestern University, at the American Astronomical Society (AAS) meeting in Washington, D.C. The results could explain, in a simple fashion, the origin of this ridge, which was discovered nearly 30 years ago.

"With our new data and existing theory, we now believe that low-energy cosmic rays bombarding and heating cold clouds of gas could be responsible for producing the X-ray emission of the galactic ridge," said Zadeh, whose work was supported by NASA. "Earlier explanations of the ridge's origin have focused on dramatic scenarios such as hot gases constantly being replenished by an unknown source as well as the possibility of an ancient explosion, but we are now offering a simpler mechanism to explain the mystery."

As its name indicates, the X-ray galactic ridge is a ridge of high-energy X-ray emission that extends along the plane of the Galaxy. The origin of the ridge has been a puzzle since its discovery in 1972.

Detailed analysis of Zadeh's new observational data indicates that a simple process such as the interaction of low-energy cosmic rays with neutral molecular clouds can explain the data without the need for unusual events such as unbound 100 million-degree gas extending throughout the nucleus of the Galaxy or an energetic explosion at the galactic center occurring approximately 300 years ago.

Zadeh's data and interpretation - that the extended and diffuse X-ray emission is produced by the collision of electrons from low-energy cosmic rays with a cloud of cold gas 1 million times the mass of the sun - are supported by a theoretical paper by scientists at NASA's Goddard Space Flight Center.

"Sometimes science does deal with simple mechanisms," said Zadeh. "Using existing theoretical work, we have shown that dramatic events are not necessary to explain the X-ray galactic ridge."

The new images have more than 10 times better resolution than previous images of areas of the X-ray galactic ridge that have been studied. Zadeh and his collaborators used NASA's Chandra X-ray Observatory to observe the X-ray emission with the best resolution currently available. Chandra, which was launched and deployed by Space Shuttle Columbia in July 1999, is the most sophisticated X-ray observatory built to date and is designed to observe X-rays from high-energy regions of the universe, such as the galactic ridge.

In addition to Zadeh, the research team included Casey Law of the Harvard-Smithsonian Center for Astrophysics and Mark Wardle of the University of Sydney, Australia. The team studied a small but interesting and well-studied portion of the ridge, located in the center of the Milky Way Galaxy and approximately 1 million times smaller than the entire ridge. Zadeh believes their study of a small portion tells a great deal about the bigger picture.

"We believe our images are a very good representation of what is happening in the entire X-ray galactic ridge," said Zadeh. "The data provides new evidence for the origin of the ridge and its X-ray emission, evidence which has implications for the origins of high-energy activity in our galaxy as well as other galaxies."

"Our results have implications for star formation in the central regions of the Galaxy," said Wardle. "The bombardment of the giant clouds by low-energy cosmic rays increases their temperature and degree of ionization, two key parameters that determine the ability of interstellar clouds to form stars."

Galactic Center Radio Arc: X-ray Gas Associated With Galactic Center Radio Arc

Galactic Center Radio Arc
Chandra observations of a region of the Galactic Center have found an X-ray filament and cloud about 40 light years across (blue). These X-ray features are associated with large filamentary and shell-like structures that are bright sources of radio waves (red).

The X-ray emission is thought to be produced when energetic electrons from the radio filaments collide with a cloud of cold gas that has a mass one million times the mass of the sun. This process of the bombardment of cold gas clouds with energetic electrons could explain the origin of the mysterious X-ray ridge along the plane of the galaxy that was discovered nearly 30 years ago.

Credit: X-ray (blue): NASA/CXC/Northwestern /F.Zadeh et al.; Millimeter Wavelength (green): Nobeyama/M.Tsuboi; Radio (red): NRAO/VLA F.Zadeh et al.
JPEG (156 k) , Tiff (2.3 MB), PS (15.6 MB)

Fast Facts for Galactic Center Radio Arc:
Credit  X-ray (blue): NASA/CXC/Northwestern/F.Zadeh et al.; Millimeter Wavelength (green): Nobeyama/M.Tsuboi; Radio (red): NRAO/VLA F.Zadeh et al.
Scale  X-ray/Molecular image is 8 by 7 arcmin. Radio image is 30 arcmin on a side.
Category  Normal Galaxy/Survey
Coordinates (J2000)  RA 17h 46m 20s | Dec -28º 52' 00.01"
Constellation  Sagittarius
Observation Date  July 7, 2000
Observation Time  13.7 hours
Obs. ID  945
Color Code  Intensity
Instrument  ACIS

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